Study Of Phase Transformations Research Articles

Dilatometric Study of Phase Transformations in Steels: Some Issues

Abstract Dilatometry is widely used in the investigation of phase transformation in steels, not only by research laboratories but also by most steel makers. Its application includes the determination of CCT and TTT diagrams, and more generally the investigation of phase transformation kinetics under controlled temperature conditions. A basic and often undiscussed assumption in the exploitation of dilatation measurements as a function of temperature is that of a constant and uniform temperature distribution in the specimen, and of a homogeneous material. In the present paper, various issues encountered with a widely used quenching dilatometer were discussed, including longitudinal temperature gradients, deformation anisotropy, or vacuum evaporation of some chemical species.

Study of Phase Transformation of Medium Carbon Steel after Hot Plastic Deformation

The CCT and DCCT diagrams of steel C60 (with approx. 0.6 % C) were constructed on the basis of dilatation tests with and/or without an influence of the previous deformation and they were then compared, order to make an evaluation of the influence of the previous deformation on the phase transformation kinetics. For the execution of the experiment, the dilatation module of the plastometer Gleeble 3800 was used. The accuracy of the diagrams was faced with metallographic analyses and measurements. The previous deformation expressly retarded a bainite transformation and slightly accelerated ferrite and pearlite transformations. The martensite start temperature was practically not influenced by the previous deformation; however, the applied deformation caused the creation of the martensite at lower cooling rates.

A study of stress-induced phase transformation and micromechanical behavior of CuZr-based alloy by in-situ neutron diffraction

The stress-induced phase transformation and micromechanical behavior of CuZr-based alloy were investigated by in-situ neutron diffraction. The pseudoelastic behavior with a pronounced strain-hardening effect is observed. The retained martensite nuclei and the residual stress obtained from the 1st cycle reduce the stress threshold for the martensitic transformation. A critical stress level is required for the reverse martensitic transformation from martensite to B2 phase. An increase of intensity for the B2 (110) plane in the 1st cycle is caused by the twinning along the {112}<111> twinning system. The convoluted stress partitioning influenced by the elastic and transformation anisotropy along with the newly formed martensite determines the microstress partitioning of the studied CuZr-based alloy. The reversible martensitic transformation is responsible for the pseudoelasticity. The macro mechanical behavior of the pure B2 phase can be divided into 3 stages, which are mediated by the evolvement of the martensitic transformation.This manuscript has been authored by UT-Battelle, LLC under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. The Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan).

The Study of Phase Transformations of AlSi9Cu3 Alloy by DSC Method

Abstract With the use of differential scanning calorimetry (DSC), the characteristic temperatures and enthalpy of phase transformations were defined for commercial AlSi9Cu3 cast alloy (EN AC-46000) that is being used for example for pressurized castings for automotive industry. During the heating with the speed of 10°C·min−1 two endothermic effects has been observed. The first appears at the temperature between 495 °C and 534 °C, and the other between 555 °C and 631 °C. With these reactions the phase transformation enthalpy comes up as +6 J g−1 and +327 J g−1. During the cooling with the same speed, three endothermic reactions were observed at the temperatures between 584 °C and 471 °C. The total enthalpy of the transitions is – 348 J g−1. Complimentary to the calorimetric research, the structural tests (SEM and EDX) were conducted on light microscope Reichert and on scanning microscope Hitachi S-4200. As it comes out of that, there are dendrites in the structure of α(Al) solution, as well as the eutectic (β) silicon crystals, and two types of eutectic mixture: double eutectic α(Al)+β(Si) and compound eutectic α+Al2Cu+β.

Experimental study of phase transformation in non-equilibrium hypoeutectic alloy from the Fe–Cr–Ni–Mo–C system

In the present study, experimental and thermodynamic analysis of phase transformation in hypoeutectic Fe–24Cr–11Ni–4Mo–0.64C alloy is presented. The investigated alloy was synthesized in an arc furnace in high-purity argon atmosphere using suction-casting unit. Chemical composition of the alloy was determined using an optical emission spectrometer. The microstructure was characterized by scanning electron microscopy. Phase composition in as-cast state was analysed by X-ray diffraction and compared with thermodynamic calculations for equilibrium state using Thermo-Calc software. The analysis of phase transformations was conducted by dilatometry and differential scanning calorimetry investigations. It was found that phase composition of the alloy after non-equilibrium solidification (cooling rate ~30 °C s−1) differs significantly from equilibrium state. Critical temperatures of phase transformations in the alloy were determined.

Study of phase transformation and elastic properties of ThX (X = N, P, As and Sb) under high-pressure

ABSTRACTAn improved interaction potential model (IIPM) has been formulated to theoretically predict the pressure induced phase transition, elastic properties and thermophysical properties of thorium monopnictides (ThX; X = N, P, As and Sb). The phase transition pressures and volume drop obtained from this model show a better agreement with the available experimental than theoretical results. We have achieved elastic moduli, anisotropy factor, Poisson's ratio, Kleinman parameter, shear and stiffness constants on the basis of the calculated elastic constants. To know the anharmonic properties, we have also computed the third-order elastic constants, first-order pressure derivatives of second-order elastic constants and thermophysical quantities. Our results are in reasonable agreement with available measured and others reported data which supports the validity of model.

Phase Transformation and Structural Characterization Studies of Aluminum Oxide (Al&lt;SUB&gt;2&lt;/SUB&gt;O&lt;SUB&gt;3&lt;/SUB&gt;) Nanoparticles Synthesized Using an Elegant Pulsed Laser Ablation in Liquids Technique

Phase Transformation and Structural Characterization Studies of Aluminum Oxide (Al&lt;SUB&gt;2&lt;/SUB&gt;O&lt;SUB&gt;3&lt;/SUB&gt;) Nanoparticles Synthesized Using an Elegant Pulsed Laser Ablation in Liquids Technique

Application of in-situ electrical resistance measurements to the study of phase transformations in ferrous alloys

Phase transformations have been studied in a variety of different steels with the use of “in situ” electrical resistance measurements. The results were evaluated by metallography of the initial and final microstructures and with consideration of data from the published literature. On this basis a good correlation has been established and it was shown that this method is suitable for such investigations. It even presents certain advantages, thus providing a more complete understanding of the physical metallurgy of steels. We out-lined the field in which the measurement of electrical resistance is particularly suitable and an example of processes that are difficult to monitor using other commonly used methods.

Study of Phase Transformations in Steel X38CrMoV5-1 Using Dilatometry and Differential Thermal Analysis

The phase transformations of the X38CrMoV5-1 (AISI H11-1) steel are studied in the present work. Parts are usually manufactured by die-casting followed by a heat treatment, and the resulting microstructure as well as the mechanical properties are highly sensitive to the heat treatment conditions. The heat treatment consists on a quenching and tempering and it is not able to remove the casting dendritic microstructure or porosity but anyway, it is carried out to achieve an acceptable ductility, good hardness, and/or tensile strength in order to avoid catastrophic failures in service. These manufactured parts are used as safety braking parts. The authors use differential thermal analysis (DTA) and dilatometry techniques to obtain the phase transformation temperatures AC1 and AC3 and, therefore achieve an optimized heat treatment for this steel application. The microstructure obtained with this optimized heat treatment is totally martensitic providing the desired mechanical properties for this application.

Thermodynamic Modeling and Experimental Study of Phase Transformations in Alloys Based on γ-TiAl

Thermo-Calc software is used to model the composition diagram for alloys based on γ-TiAl of the systems Ti – Al – Mo – (4 – 10) at.% Nb and Ti – Al – Nb – X (X is Cr, Mo, V). The effect of alloying on critical points and sequence of phase transformations is established. Changes in phase composition in relation to alloy TNM-B1 temperature are analyzed using a polythermal section of the Ti – Al – Nb – Mo system.

The Microstructure and Phase Composition of 35CrSiMN5-5-4 Steel After Quenching and Partitioning Heat Treatment

Abstract The aim of the study was to characterise the microstructure of 35CrSiMn5-5-4 steel which was subjected to a new heat treatment technology of quenching and partitioning (Q&amp;P). The parameters of the treatment were chosen on the basis of computer simulations and dilatometric studies of phase transformations occurring in steel. The transmission electron microscopy (TEM) observations of steel microstructure after the Q&amp;P treatment revealed the presence of martensite as well as significant amount of retained austenite in form of layers between the martensite laths. The rod-like carbides in the ferritic areas were also observed, which indicates the presence of lower bainite in steel. It was found that the retained austenite content measured by means of TEM was about 28% for partitioning at 400°C and 25% for partitioning at 260°C. These results are in good agreement with the phase composition calculated theoretically as well as those determined experimentally by use of dilatometric tests.

Raman study of phase transformation from diamond structure to wurtzite structure in the silicon nanowires

Hexagonal silicon has emerged as an exciting material due to its novel vibrational and electronic properties. Synthesis of the wurtzite silicon nanowires (w-SiNWs) is studied here using metal assisted chemical etching (MACE) technique. Stress induced in the SiNWs during wet chemical etching is attributed to formation of the w-SiNWs. Presence of the w-SiNWs is revealed by first-order and second-order Raman spectra. The effect of variation of deposition time of silver (catalyst) is explicitly studied for growth of w-SiNWs. The deposition time enhances the density of SiNWs in an island of vertically aligned SiNWs. Absorption coefficient studies of the w-SiNWs are also conducted using UV–vis spectroscopy as a function of deposition time. Increase in the absorption coefficient in SiNWs is noticed with increasing deposition time. The prominent quantum confinement along with stress and porosity is shown to be mainly responsible for the transformation from diamond structure to wurtzite structure in the silicon nanowires.

Effective behavior of an interface propagating through a periodic elastic medium

We consider a moving interface that is coupled to an elliptic equation in a heterogeneous medium. The problem is motivated by the study of displacive solid-solid phase transformations. We argue that a nearly flat interface is given by the graph of the function g which evolves according to the equation g_t (x) = -(-\Delta)^{1/2}g (x) + \varphi(x, g(x)) + F where -(-\Delta)^{1/2}g describes the elasticity of the interface, \varphi(x, g(x)) the heterogeneity of the media and F the external force driving the interface. This equation also arises in the study of ferroelectric and ferromagnetic domain walls, dislocations, fracture, peeling of adhesive tape and various other physical phenomena. We show in the periodic setting that such interfaces exhibit a stick-slip behavior associated with pinning and depinning. Specifically, there is a critical force F^\star below which the interface is trapped, and beyond which the interface propagates with a well-described effective velocity that depends on F . We present numerical evidence that the effective velocity ranges from v \sim (-\log |F-F^\star|)^{-1} to (F-F^\star)^\beta for some 0&lt;\beta\le1 for F close to F^\star depending on \varphi . We obtain \beta = 1/2 for the case of non-degenerate smooth obstacles. We further present numerical evidence that F^\star can depend on direction and sense of propagation.

Dislocation-mediated plasticity in silicon during nanometric cutting: A molecular dynamics simulation study

The nucleation and propagation of dislocations and its consequence on the defect structure in silicon during nanometric cutting are not well known, although the amorphization and high pressure phase transformation studies on silicon have remained at the epicentre of research across various disparate disciplines for over a decade. This paper proposes a new mechanism of crystal plasticity identified by a fully automated dislocation extraction algorithm in molecular dynamics simulations of nanometric cutting of silicon for different cutting planes/directions at a wide range of temperatures (300–1500K). Alongside amorphization of silicon, our simulations revealed nanoscale stochastic nucleation of dislocations and stacking faults, which serve as mediators of microscopic plasticity during various contact loading operations and manufacturing processes of silicon. Of interest is that, irrespective of the cutting temperature, the stacking faults, which were not formed for either the (010)[100] or (111)[1̅10] crystal setups, were generated with three atomic layers in the (110)[001̅] cutting.

Atomistic studies of shock-induced phase transformations in single crystal iron with cylindrical nanopores

Non-equilibrium molecular-dynamic simulations with our modified analytic embedded-atom model potential were carried out on single crystalline iron of idealized cylindrical nanopores to study the effect of defect on shock response. The results showed that the shock response and phase transformation were influenced by cylindrical nanopores. Reflection wave, “hot spot” can be clearly observed under [001], [110] and [111] loading directions, and the nucleation and growth of dislocations appeared for the shock along [110] and [111] directions. The critical stress of phase transformation, the nucleation sites and martensitic variants were obviously influenced by the cylindrical nanopores. In addition, stress assisted transformation mechanisms and strain induced transformation mechanisms were discussed, and the kinetics of the variants which caused by the presence of cylindrical pores were studied.

XAFS characterization of industrial catalysts: in situ study of phase transformation of nickel sulfide

The online sulfiding process for nickel-contained catalyst often ends up with a nickel sulfide mixture in refinery plant. To elucidate the local environment of nickel and its corresponding sulfur species, a model catalyst (nickel sulfide) and model thermal process were employed to explore the possibilities for characterization of real catalysts in industrial conditions. The present investigation shows effectiveness of in situ XANES and EXAFS measurements for studying the phase stability and phase composition in these systems, which could be used to simulate real sulfiding process in industrial reactions, such as hydrodesulfurizations of oil.

A Dilatometric Study of the Non Isothermal Aging of a Cold Rolled Al-Mg-Si Alloy

In contrast to isothermal aging, few reports document the non-isothermal aging of deformed Al–Mg–Si alloys. The knowledge of non-isothermal aging of pre-deformed Al–Mg–Si alloys is of primary importance to understand the thermal stability as well as to control the microstructure of the final product during industrial processing. Therefore, the present work has been focused to understand the microstructure evolution during the continuous heating of a cold rolled Al–Mg–Si alloy. This has been followed using dilatometry, Differential Scanning Calorimetry, X-Ray Diffraction and microhardness measurement. Based on the results obtained, it is shown that dilatometry is a powerful tool to study phase transformations in deformed Al-Mg-Si alloys, moreover, the microstructural evolution, of the cold rolled sample, can be described as follows: at the earlier stages of the non-isothermal aging, formation and then the reversion of fully coherent GP zones take place. This is followed by the simultaneous occurrence of β” and β’ precipitation and recovery reaction. By continuing aging, the next reactions which will take place are β” and β’ dissolution and recrystallization. Finally, one can observe the formation and then the dissolution of the equilibrium phase β.

A study of densification and phase transformations of nanocomposite Cu-Fe prepared by mechanical alloying and consolidation process

The purpose of this study is an attempt to obtain nanocrystalline bulk Cu-40 at.% Fe alloy by mechanical alloying followed by consolidation process under 7 GPa and annealing. The transformations occurring in the material were studied by the use of X-ray diffraction. The structural investigations revealed that consolidation enabled supersaturated Cu(Fe) to precipitate out Cu4Fe and CuFe4 intermetallics. A limited grain growth was noticed after annealing. The results of microhardness have shown that the hardening increases similarly up to 40 h milling. The variation of microhardness as a function of the reciprocal square root of the grain size demonstrates that the rate of hardening is similar to that predicted by the Hall-Petch behavior.

Effect of Nanosize Yittria and Tungsten Addition to Duplex Stainless Steel During High Energy Planetary Milling

In this present investigation, elemental powders of duplex stainless steel composition (Fe-18Cr-13Ni) with 1 wt. % nano yittria and tungsten were milled separately in dual drive planetary mill (DDPM) for 10 h to fabricate yittria dispersed and tungsten dispersed duplex stainless steel powders. The milled powder samples were characterized by X-Ray diffraction and scanning electron microscopy (SEM) to study the size, morphology and phase evolution during milling. The gradual transformation from ferrite to austenite is evident from XRD spectra during milling. The crystallite size and lattice strain of yittria dispersed duplex stainless steel after 10 h milling were found to be 7 nm and 1.1% respectively. The crystallite size of tungsten dispersed duplex stainless steel was 5 nm. It has been observed from SEM analysis that particles size has been reduced from 40 to 5 μm in both cases. Annealing of 10 h milled powder was performed at 750°C for 1 h under argon atmosphere to study phase transformation in both yittria and tungsten dispersed duplex stainless steel. The XRD analysis of annealed stainless steel depicts the phase transformation from α-Fe to γ-Fe with the formation of oxides of Y,Fe and Cr. The differential scanning calorimetry analysis was conducted by heating the milled powder from room temperature to 1200°C under argon atmosphere to investigate the thermal analysis of both the stainless steel powders.

A first-principles study of pressure-induced phase transformation in a rare-earth formate framework.

Among the panoply of exciting properties that metal-organic frameworks (MOFs) exhibit, fully reversible pressure-induced phase transformations (PIPTs) are particularly interesting as they intrinsically relate to the flexibility of MOFs. Recently, a number of MOFs have been reported to exhibit this feature, which is attributed to bond rearrangement with applied pressure. However, the experimental assessment of whether a given MOF exhibits PIPT or not requires sophisticated instruments as well as detailed structural investigations. Can we capture such low pressure transformations through simulations is the question we seek to answer in this paper. For this, we have performed first-principles calculations based on the density functional theory, on a MOF, [tmenH2][Y(HCOO)4]2 (tmenH2(2+) = N,N,N',N'-tetramethylethylenediammonium). The estimated lattice constants for both the parent and product phases of the PIPT agree well with the earlier experimental results available for the same MOF with erbium. Importantly, the results confirm the observed PIPT, and thus provide theoretical corroborative evidence for the experimental findings. Our calculations offer insights into the energetics involved and reveal that the less dense phase is energetically more stable than the denser phase. From detailed analyses of the two phases, we correlate the changes in bonding and electronic structure across the PIPT with elastic and electronic conduction behavior that can be verified experimentally, to develop a deeper understanding of the PIPT in MOFs.